The present invention has a particularly advantageous, yet non-limiting application, in the case of so-called “narrowband” useful signals for which the spectral bandwidth is typically less than 2 kilohertz, or even less than 1 kilohertz.
In the case of useful narrowband signals, numerous phenomena can cause a frequency drift that is greater than the spectral bandwidth of said useful signals.
Firstly, since the useful signals are received by a receiving station, relative movements of the terminals in relation to said receiving station can result, by Doppler effect, in frequency drifts which can be significant with regard to the spectral bandwidth of the useful signals if the relative travelling speeds are high. Such frequency drifts can be caused by mobility of the terminals and/or mobility of the receiving station.
In particular, in the case of a receiving station on board a satellite in moving orbit, for example a LEO low earth orbit, the speed of travel of said satellite relative to the terminals (fixed or mobile) can result in significant frequency drifts affecting the useful signals received by said receiving station.
The frequency drift can also be caused during emission in an uncontrolled manner. More specifically, the emission of a useful signal requires a terminal equipped with frequency synthesis means (local oscillator, mixer, etc.) implemented for the frequency translation of the useful signal, generated as a baseband signal, to a frequency band for multiplexing the useful signals. Such frequency synthesis means create a frequency drift which, in particular in the case of useful narrowband signals, can be significant relative to the spectral bandwidth of said useful signals, in particular in the case of low-cost terminals for which the frequency synthesis means are inefficient. However, the frequency drift caused by the frequency synthesis means is low compared to that caused by the relative movements of the terminals with regard to the receiving station, in particular over a short period of time such as the duration of a useful signal.
To a lesser degree, the frequency synthesis means of the receiving station can also create a frequency drift.
The detection of such useful signals is complex, all the more so since the frequency drift can vary from one useful signal to another. Moreover, the initial receiving frequency and the time of receipt of each useful signal are not necessarily known, which makes the detection of the useful signals even more complex, since they must be detected in a blind or near-blind manner a priori without knowing the frequency drifts, the times of receipt and the initial receiving frequencies.
Maximum likelihood estimators can be implemented, which test all possible possibilities. However, the quantity of calculations to be carried out and the quantity of data to be stored in memory is in this case very high, and can be prohibitive, in particular for the detection of useful signals at a satellite.